Our previously-reported work on OCA1A and OCA1B mouse models has been published in the Journal of Clinical Investigation. This work established the nitisinone could increase melanin pigmentation in a mouse model of OCA1B, but not OCA1A. Our work in the past year has focussed on three questions: 1) Does nitisinone treatment in the Himalayan (OCA1B) mouse affect retinal function? 2) Can nitisinone improve melanin pigmentation in other mouse models of OCA? and 3) Will nitisinone treatment improve pigmentation in humans with OCA1B. We have evaluated the effect of 1 month nitisinone treatment on the full-field electroretinogram (ERG) of Himalayan mice. This test evaluates overall photoreceptor- and post-photoreceptor function in the neurnal retina, as well as, to some extent, the function of the retinal pigment epithelium. It does not measure visual acuity, which is poor in mice and would not be anticipated to change with treatment. Although our analysis is ongoing, present data indicate that nitisinone does not affect any of the ERG parameters (a wave, b wave, c wave) measured. We have obtained mouse models of OCA3(brown) and OCA4(underwhite) mice. OCA3 mice have brown fur and eye color. One month of nitisinone did not result in a visible change in melanin pigmentation in these mice; however, the final result will depend on electron microscopic analyis of ocular tissues. OCA4 mice were slower to breed and we have only now approached the proper number of mice to proceed. Lastly, the NEI ICRRC has approved a protocol outline for a pilot study of nitisinone in adults with OCA1B. We have written a full clinical protocol and are awaiting co-investigator input prior to submitting it for approval to the ICRRC. One of the complex issues addressed in our work was the development of appropriate clinical outcome measures. Since we do not anticipate that nitisinone will affect foveal function (ergo, visual acuity) in a mature (adult) retina, we are using ocular melanin pigmentation as our primary outcome variable. We have developed a semi-quantitate grading scale of iris transillumination (our primary outcome variable) and are submitting this work for publication.